To obtain water-soluble or water-dispersible polymers including hydrophobic blocks, one currently known method is that known as “micellar radical polymerization”. Examples of micellar polymerization have been described especially in U.S. Pat. No. 4,432,881 or in Polymer, vol. 36, No. 16, pp. 3197-3211 (1996), to which reference may be made for further details.
According to the particular technique of “micellar radical polymerization”, which will be referred to as “micellar polymerization” for the purposes of brevity in the rest of the description, block polymers of multiblock type are synthesized by copolymerization of hydrophilic monomers and hydrophobic monomers in an aqueous dispersing medium (typically water or a water/alcohol mixture) which comprises:                hydrophilic monomers in dissolved form or dispersed in said medium; and        hydrophobic monomers in surfactant micelles formed in said medium by introducing therein this surfactant at a concentration above its critical micelle concentration (cmc).        
According to a particular embodiment, the hydrophobic monomers present in the surfactant micelles used in micellar polymerization may be monomers, which themselves have the property of forming micelles without the need to add additional surfactants (monomers referred to as “self-micellizable” in the rest of the description). According to this particular embodiment, the surfactant used may be the self-micellizable hydrophobic monomer itself, used without any other surfactants, although the presence of an additional surfactant of this type is not excluded. Thus, for the purposes of the present description, when mention is made of hydrophobic monomers in surfactant micelles, this notion encompasses both (i) hydrophobic monomers present in surfactant micelles other than these monomers, and (ii) monomers comprising at least one hydrophobic part or block and forming by themselves the micelles in aqueous medium. The two abovementioned modes (i) and (ii) are compatible and may coexist (hydrophobic monomers in micelles formed with another self-micellizable monomer, for example, or alternatively micelles comprising a combination of surfactants and of self-micellizable monomers).
In micellar polymerization, the hydrophobic monomers contained in the micelles are said to be in “micellar solution”.
The micellar solution to which reference is made is a micro-heterogeneous system that is generally isotropic, optically transparent and, thermodynamically stable.
It should be noted that a micellar solution of the type used in micellar polymerization should be distinguished from a microemulsion. In particular, in contrast with a microemulsion, a micellar solution is formed at any concentration exceeding the critical micelle concentration of the surfactant used, with the sole condition that the hydrophobic monomer is soluble at least to a certain extent in the internal space of the micelles. A micellar solution moreover differs from an emulsion by the absence of an internal homogeneous phase: micelles contain a very small number of molecules (typically less than 1000, in general less than 500 and typically from 1 to 100, most usually with 1 to 50 monomers and not more than a few hundred surfactant molecules when a surfactant is present) and the micellar solution generally has physical properties similar to those of the surfactant micelles without monomers. Moreover, usually, a micellar solution is transparent with respect to visible light, given the small size of the micelles, which do not bring about refraction, unlike the drops of an emulsion, which refract light and give it its characteristic cloudy or white appearance.
The micellar polymerization technique leads to characteristic block polymers which each contain several hydrophobic blocks of substantially the same size, in which this size may be controlled. Specifically, given the confinement of the hydrophobic monomers in micelles, each of the hydrophobic blocks formed, of controlled, size, contains substantially a defined number nH of hydrophobic monomers, this number nH being able to be calculated as follows (Macromolecular Chem. Physics, 202, 8, 1384-1397, 2001):nH=Nagg·[MH]([surfactant]−cmc)where:                Nagg is the aggregation number of the surfactant, which reflects the number of surfactants present in each micelle        [MH] is the molar concentration of hydrophobic monomer in the medium, and        [surfactant] is the molar concentration of surfactant in the medium        cmc is the critical micelle (molar) concentration.        
The micellar polymerization technique thus enables advantageous control of the hydrophobic units introduced into the formed polymers, namely:                overall control of the mole fraction of hydrophobic units in the polymer (by modulating the concentration ratio of the two monomers); and        a more specific control of the number of hydrophobic units present in each of the hydrophobic blocks (by modifying the parameters influencing the nH defined above).        
The multiblock polymers obtained by micellar polymerization also have an associative nature, which makes them, in absolute terms, good candidates for applications as viscosity enhancers.
However, as a general rule, micellar polymerization leads to polymer chains of highly non-homogeneous size. Specifically, although they allow fine control of the size of the hydrophobic blocks, the current micellar polymerization techniques lead to more erratic polymerization of the hydrophilic monomer units. Thus, the micellar polymerization processes currently known lead to polymer chains often comprising high heterogeneity in terms of composition, with, as a general rule, a highly polydispersed molecular mass distribution, without possible predefinition of the mean molecular mass Mn obtained. This inhomogeneity is especially revealed in the abovementioned article Polymer, vol. 36, No. 16, pp. 3197-3211 (1996).
Furthermore, the microstructure of the polymers obtained (i.e. the distribution of the hydrophobic blocks in the various chains) is not controlled, which is due especially to the very short lifetimes of the propagating chains relative to the overall polymerization time, combined with the differences in reactivity of the propagating active centres with respect to the hydrophilic monomers and the hydrophobic monomers (and also to their differences in concentration).
In other words, micellar polymerization does indeed make it possible, in the most general case, to integrate hydrophobic blocks of controlled size into hydrophilic chains, which makes it possible to synthesize self-associative polymers, but without controlling the overall size of the polymers synthesized or the microstructure of these polymers, which does not allow fine control of the properties of these self-associative polymers.
Moreover, the absence of control of the microstructure does not allow sufficiently fine modulation and control of the properties of polymers synthesized by micellar polymerization. Furthermore, it prevents access to copolymers of controlled architecture.
In addition, micellar polymerization processes are generally limited to extremely dilute systems to enable the addition and mixing of reagents. The molecular masses obtained in micellar radical polymerization are generally of the order of 500 000 to 5 000 000 g/mol, for example from 500 000 to 3 000 000.
To reduce the composition drift in polymers derived from micellar polymerization, a process of semi-continuous addition of hydrophobic monomers was proposed in U.S. Pat. No. 6,207,771. Although interesting, this process does not, however, allow effective control of the microstructure and does not at all control the molecular masses.
One aim of the present invention is to provide block polymers comprising hydrophobic blocks of controlled size, of the type obtained in standard micellar polymerization, but improving the control of the mean molecular mass of the chains synthesized and also allowing control of the microstructure of polymers, i.e. homogeneity, from one polymer chain to another, of the distribution of the hydrophobic blocks in the hydrophilic backbone.